The invention relates to a method and apparatus for manufacturing a discharge lamp provided with a discharge vessel which encloses a discharge space with a ceramic wall, said discharge vessel being obtained through sintering of a body formed in a sludge molding process, which sludge molding process comprises injecting a porous outer mold with sludge, deposition of sludge particles against the injected outer mold, removing excess sludge, removing the outer mold, and pre-firing of the molded body.
The invention also relates to a lamp provided with a discharge vessel with a ceramic wall.
This type of method is known from EP 0926106. The sludge is a liquid suspension of sludge particles in a suspension liquid. Water is generally used as the suspension liquid. A sludge molding process is understood to involve, in the context of the present description and claims, a process in which a closed space surrounded by the porous outer mold is filled with sludge, the sludge particles are deposited against the mold wall owing to a removal of suspension liquid, and the remaining, still liquid sludge is decanted. The removal of suspension liquid may take place here both by means of capillary absorption of the suspension liquid by the outer mold and by means of a forced escape of suspension liquid through an applied pressure difference between the enclosed space and the surroundings of the outer mold.
The method allows the formation of a discharge vessel in which a portion designed for enclosing a discharge space and portions designed for accommodating electrical lead-through members are formed as one integral whole. The risk of a sintering seam, which forms the connection between different portions of the discharge vessel, becoming leaky is eliminated thereby.
In practical lamps, the discharge vessel is provided with a discharge space portion which is enclosed by the ceramic wall and has projecting closures at mutually opposed ends, which closures are formed as elongate tubular plugs each with a free end through which the electrical lead-through member is passed to an electrode positioned in the discharge space. Each plug is closed at its free end by means of a suitable melting glass or melting ceramic. The melting glass or melting ceramic also provides an adhesion between the plug and the associated lead-through element.
A disadvantage of the known method is that it leads to the formation of practical lamps with a comparatively great external diameter of the plugs. This may give rise to an undesirable heat balance of the manufactured lamp. A further disadvantage is that an after-treatment of the plug is often found to be necessary for realizing a suitable lead-through opening. The after-treatment consists, for example, in reaming of the plug of the molded discharge vessel, possibly followed by a polishing treatment. After-treatments are disadvantageous because they make the lamp manufacturing process more complicated and also because they increase the risk of production wastage.